Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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Nuclear, Stellar Disks in E/S0 Galaxies

W. Jaffe
Leiden Observatory, P.B. 9513, Leiden, 2300 RA, Netherlands

F. C. van den Bosch
Leiden Observatory, P.B. 9513, Leiden, 2300 RA, Netherlands

H. C. Ford
Johns Hopkins University, Dept. of Physics and Astronomy, Baltimore, MD, 21218, USA

Abstract:

A considerable fraction of S0s and fainter Ellipticals harbor a small, bright, stellar disk in their nucleus. The brightness and simple rotation of these disks allow better determination of the central mass than in ellipticals with no disks. NGC 4570, for example, seems to have a central black hole of .

Keywords: Galaxies, nuclei

Introduction

Our WFPC1 survey of a complete sample of Virgo Cluster E and E/S0 galaxies revealed the presence of bright, thin stellar disks in the nuclear regions of several of them (Jaffe et al. 1994, van den Bosch et al. 1994, Ferrarese et al. 1994). These nuclear disks have scalelengths of --30 pc, and have central surface brightnesses of mag/arcsec. The ellipsoidal components in which the disks are embedded have steep central cusps with (Scorza & van den Bosch 1996). In all cases where we find nuclear disks, an outer disk is clearly present as well, suggesting that these galaxies are S0s rather than genuine ellipticals.

As shown in Table 1, at least three of the 14 galaxies surveyed have such disks while in four cases, dust in the nucleus makes detection impossible. Rix & White (1990) showed that disks become photometrically undetectable for inclination angles smaller than . The three nuclear disks in our sample are inclined to the line-of-sight by more than 80 degrees. If viewed face-on, they would be a magnitude or two fainter. This suggests that some of the rounder galaxies, where we see no disk, contain face--on disks.

 
Table 1:

Nuclear Disks and S0 Galaxies

We see that many or most of the highly flattened, fainter galaxies have nuclear disks. Are we really just seeing the disks of S0 galaxies extended into the nuclear regions? The galaxies in which we see disks are variously classified as E/S0 or E6--7. Van den Bergh (1990) has shown from the distribution of ellipticities of E and S0 galaxies that almost all faint E galaxies are actually S0s. In most cases where we see a nuclear disk, we see evidence for a substantial disk at larger radii. So it is reasonable to conclude that the nuclear disks are simply part of S0 structures.

On the other hand, the nuclear disk is clearly distinct for the main S0 disk. In the best cases, such as NGC 4570 (c.f. fig 9 in van den Bosch et al. 1994), the nuclear disk has a sharp outer edge at about 100 pc radius, and there is a clear gap before the inner edge of the main disk begins at about twice this radius. The nuclear disk is also much thinner than the outer disk. We can ask whether the two disks are distinct in origin, kinematics, and population, or only separated by a spatial gap caused by a dynamical instability.

The original WFPC1 images were monochromatic and give no answer to this question. We now have scheduled, and partly completed, HST observations that should be adequate. We will get U, V, and I, band photometry of NGC 4342 and NGC 4570, with the WFPC2, as well as FOS spectroscopy at various positions along the nuclear disks. With only partial results in hand, we find the nuclear disks to be redder by magnitudes in U-V than the bulge.

Nuclear Disks and Elliptical Galaxies

Our work, as well as that by Lauer et al. (1996) on a larger sample of galaxies, indicate that nuclear disks are present only in galaxies that have steep central density cusps. These are the fainter galaxies, that are often found to be disky. It has been argued in the literature that these faint ellipticals form a smooth transition from the S0s going to smaller disk-to-bulge ratios (e.g., Scorza & Bender 1995). Since total luminosity does not change with inclination, we believe that the absence of disks in the bright ellipticals is not a projection effect but a real difference. This view is supported by other differences between the bright and faint galaxies, particularly that the fainter ones are supported by rotation while the bright ones are supported by anisotropic pressure.

Where does this difference arise? On the one hand, the bright and faint galaxies may have been formed in different environments, where the angular momentum distributions are different. On the other, the bright galaxies may be the product of order--destroying mergers of the fainter galaxies. Part of the answer will come from the the same color and population material that we are now collecting, which will tell us if the centers of the disk galaxies differ from those of the true ellipticals. Another approach is dynamic modelling: use N-body codes to see whether collisions of a number of S0 galaxies look like a plausible elliptical galaxy. Numerous N-body simulations have already shown that two merging spirals result in genuine, giant ellipticals (e.g., Barnes 1988). However, to date, the N-body codes used lack the resolution to study the merging of the high density cores of the faint ellipticals and S0s.

Central Masses in Faint Galaxies

Although posing a new set of questions, the nuclear disks help provide an answer to a major existing question: whether massive black holes inhabit the centers of all early type galaxies, or only the very bright ellipticals associated with AGNs.

Proving the presence of a BH in the nucleus of elliptical galaxies is complicated by the fact that the 3-dimensional velocity distribution of elliptical galaxies can be strongly anisotropic. Disks, however, are cold components, strongly dominated by rotation. Furthermore, they are generally thin so that interpretation of the line-of-sight velocity distribution is far less complicated than for the cores of ellipticals. Therefore, nuclear disks embedded in the nuclei of elliptical galaxies offer a promising means of detecting nuclear BHs. To date, the best cases for black holes in elliptical galaxies are based on emission lines from gaseous disks rather than from stellar dynamics (Harms et al. 1994, Ferrarese, Ford, & Jaffe 1996). However, the interpretation of the kinematics of gas disks can also be complicated, especially in AGNs where outflow, inflow, and turbulent phenomena are very likely to be play a major role. Since the motion of stars is purely gravitational, stellar disks do not suffer from these effect.

Van den Bosch & de Zeeuw (1996) constructed two-integral models of ellipticals with central density cusps and embedded nuclear disks. They calculate the line-of-sight velocity distributions of these models in order to investigate to what extent the presence of a nuclear stellar disk can contribute to an accurate determination of the central density of these systems. They find that, for sufficient spatial and spectral resolution, the rotation of the nuclear disk can be accurately measured, providing an accurate measure of the central mass-to-light ratio that is free of the velocity anisotropies that hamper the interpretation of a central increase in velocity dispersion. The nuclear stellar disks, therefore, offer one of the best laboratories for testing the presence of massive BHs. They are very bright and very ``cold'', apparently in nearly circular motion. Thus with high spatial resolution spectra, obtainable with the FOS aboard the HST, we can measure the rotation velocities near the center and infer whether a massive BH is present.

We have preliminary results for NGC 4570 based on our old WFPC1 photometry, which gives the mass distribution of the stars, ground-based spectroscopy from the William Herschel Telescope, which gives the kinematics at large distances, and finally with new HST/FOS spectroscopy of the inner arcsec. The WHT data show a sharp central increase of the velocity dispersion, from km s at up to 210 km s in the center. The rotation velocity is essentially flat at 100 km s from from the center (700 pc) into about 3. The HST spectra reveal that the rotation velocity is still high, 120 km s, at 0.'' 5 from the center, while the dispersion at the nucleus is 280 km s.

We have constructed two-integral models of the stellar motions. With the mass/light ratio and central mass as free parameters we force the models to fit the HST surface brightness distribution and our measured rotations and dispersions. We have simultaneously fit the WHT and HST data, by taking the convolution with the proper PSF into account. We can not fit the HST data without a BH, since these models predict a rotation velocity at 0.'' 25 from the center of only 45 km s, whereas we measure 80 km s. Also, this model predicts a central velocity dispersion of only 160 km s. However, we do find satisfactory fits to both the WHT and the HST data after adding a central BH of . This is about an order of magnitude less than the masses inferred in M 87 and NGC 4261 (Harms et al. 1994, Ferrarese, Ford, & Jaffe 1996) which is in rough proportion to the luminosity of the host galaxies. This would support the suggestion of Kormendy & Richstone (1995) that, in general, the central BH mass is proportional to the total galaxy mass.

The data soon in hand will confirm whether this result is repeated in several of the other galaxies where we found nuclear, stellar disks.

Conclusions

Most faint, early type galaxies are actually S0s, and most have nuclear disks. Nuclear, stellar disks are ideally suited to determine the central mass density of the galaxies in which they are embedded. Nuclear massive black holes are not the sole prerogative of bright active galaxies.

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